The present invention relates to cellular communication systems. More particularly, the present invention relates to a cellular communication system providing improved handoffs between cellsites.
Conventional cellular telephone systems provide radio communications over a service area using an allocated frequency band divided into radio channels. The service area is divided geographically into cells. Each cell contains an associated cellsite providing radio communications for the cell. The cellsites include, inter alia, a radio transmitter and a radio receiver for establishing radio communications with a mobile telephone. Each cellsite is coupled to a system controller, such as a mobile telephone switching office (MTSO), either directly or through a base station (cellsite) controller.
In conventional cellular telephone systems, handoffs allow a call to continue as a mobile telephone crosses the boundary between two cells. FIG. 1 illustrates the sequence of a conventional handoff. For purposes of simplicity, it is assumed that the cellsites are connected directly to the MTSO. It is understood, however, that an analogous handoff sequence would apply in systems employing base station controllers.
Referring to FIG. 1, a handoff from one cellsite to another is initiated when the cellsite handling the call (the serving cellsite) determines that the received signal strength from the mobile telephone falls below a predetermined threshold value. (Step 105). A low signal strength indication implies that the mobile telephone is near the cell's border. When the signal level falls below the predetermined threshold value, the serving cellsite sends a handoff request to the MTSO. (Step 110).
In response, the MTSO sends "locate measurement requests" (Step 115) to the serving cellsite and its neighboring cellsites to determine which cellsite receives the mobile telephone's signal with the best signal strength. In response, the neighboring cellsites employ scanning receivers to determine the signal strength from the mobile telephone on the specified channel. The neighboring cellsites, as well as the serving cellsite, send "locate measurement responses" to the MTSO. (Step 120). If one of the neighboring cellsites reports a better signal level to the MTSO, a handoff to that cellsite (the target cellsite) will be attempted.
To attempt a handoff, the MTSO sends a new channel setup request to the target cellsite. (Step 125). In response, the target cellsite acknowledges the new channel set up request indicating that the new channel is ready to receive the mobile telephone. (Step 130). The MTSO then initiates handoff by sending a handoff order to the serving cellsite. (Step 135). The serving cellsite, in response, sends a handoff order to the mobile telephone commanding it to switch from the current channel to the new channel. (Step 140). The mobile telephone then sends a handoff order acknowledgment to the serving cellsite. (Step 145). The serving cellsite sends an acknowledgment to the MTSO (Step 150) and, upon receiving the acknowledgment, the MTSO switches the call from the serving cellsite to the target cellsite and releases the old channel (Step 152).
Meanwhile, after the mobile telephone sends the handoff order acknowledgment (Step 145), it switches to the second channel. Upon arriving, the mobile telephone sends a "mobile on channel" signal to the target cellsite (Step 155), which in turn, sends a "mobile on channel" signal to the MTSO (Step 160). If the MTSO did not previously receive the handoff order acknowledgment (e.g., because of fading or distortion), the MTSO switches the call from the serving cellsite to the target cellsite and releases the old channel (Step 152) upon receiving the "mobile on channel" signal.
During portions of the handoff sequence described in FIG. 1, system users experience periods of muting while their call is handed off from the serving cellsite to the target cellsite. As system operators reconfigure their systems to meet increasing demand by shrinking cells, instituting sectorization, and deploying microcells, the frequency of handoffs and resultant muting increases, especially in densely populated areas. Communications can significantly denigrate depending on the duration of the muting. For example, in communications involving data transfer, interruptions caused by muting can affect reliability, forcing some mobile users to employ special modem equipment to ensure data integrity. Moreover, even in ordinary conversations, gaps of over 250 milliseconds are generally noticeable to most subscribers.
In a conventional handoff, a mobile telephone's speech is muted from the time the mobile telephone receives a handoff order (Step 140) until the mobile telephone finishes transmitting a "mobile on channel" signal (Step 155). The duration of this muting ranges from about 100 milliseconds in Time Division Multiple Access (TDMA) systems up to about 250 milliseconds in Advanced Mobile Telephone (AMPS) systems. Because this period is determined by characteristics of the mobile telephone itself (e.g., the radio interface standard employed and speed of the frequency synthesizer), little can be done to shorten this period without using special mobile equipment.
On the switch side, speech is muted from the time the mobile telephone receives a handoff order (Step 140) until the mobile telephone finishes transmitting a "mobile on channel" signal (Step 155) or the network path is switched in response to a handoff order acknowledgment (Step 150), whichever is later. If the MTSO receives no handoff acknowledgment (e.g., because of fading or distortion), speech is muted until the network path is switched in response to the "mobile on channel" signal or an indication from the old channel that the mobile is no longer present, whichever is later. If the mobile arrives on the new channel before the network path is switched, the muting period imposed by the mobile telephone itself may be extended by 150 milliseconds or more.
U.S. Pat. No. 5,101,501 to Gihousen et al., assigned to Qualcomm Incorporated, describes a Code Division Multiple Access (CDMA) cellular system capable of reducing certain muting during handoffs by using a "soft handoff" technique. According to this technique, the serving and target cellsites monitor the same frame of the same channel and, on a frame-by-frame basis, a selector evaluates both frames and chooses the frame with the highest quality. This is possible in a CDMA system because adjacent cellsites can use the same radio channel and thus simultaneously monitor the same transmission of a mobile telephone. Likewise, in the reverse direction, both the serving and target cellsites simultaneously transmit to the mobile telephone. Although this soft handoff technique is effective in reducing certain muting during handoffs, it relies on CDMA infrastructure and coding, and is unworkable in existing AMPS and TDMA systems.
U.S. Pat. Nos. 5,327,577 and 5,109,528 to Uddenfelt, assigned to Telefonaktiebolaget L. M. Ericsson, are directed to handoff techniques which reduce muting in a cellular communication systems. The systems of these patents, however, rely in large part on an adaptive channel allocation system wherein at least some radio channels are common to neighboring cellsites. Thus, like the CDMA system, these techniques are generally incompatible with existing AMPS and TDMA systems.
To diminish muting problems, some existing AMPS systems use conference bridges which combine speech via the target and source cellsites during handoffs. While reducing muting due to lost handoff acknowledgments, the action of switching the conference bridge into and out of the call, in itself, breaks the network path causing brief muting before and after the handoff. Moreover, conferences bridges are generally suitable only for AMPS systems since they do not support coded speech, such as Vector Sum Excited Linear Prediction (VSELP) coding used in TDMA systems. Conference bridges also represent a significant investment of specialized equipment, and must be engineered to support peaks of handoff activity without lowering the grade of service to mobile subscribers.
Accordingly, there remains a need for a cellular communication system and method providing improved handoff capability that reduces muting during handoffs between cellsites and is compatible with existing cellular telephone systems and standards, without incurring the drawbacks associated with conference bridges.